Study On Current Leads And Pulse Tube Cooler For Small Superconducting Magnet

Recent progresses in the development of small cryocooler and the discovery ofhigh temperature superconducting materials have improved the possibility of usingthe cryocoolers-cooled low temperature superconducting magnet system. Pulse tubecooler has no moving parts at cold head, thus it has been considerable advantages ofhigher reliability, lower mechanical vibration, lower noise and longer lifetime thanother types of refrigerators. This paper introduced a small NbTi superconductingmagnet system cooled by a self-made two-stage pulse tube cooler, and a pair of binarycurrent leads was also designed for this system. The results of the experiments on thefirst stage of the pulse tube cooler are presented. The detailed research work isfollowed:1. Manufacture and assemble the components of the system that the small NbTisuperconducting magnet is cooled by two-stage pulse tube cooler. Thesuperconducting magnet system, the cryogenic system and other accessorialcomponents are introduced in detail.2. Design of a pair of binary current leads. The current leads are consisted ofnormal copper at high temperature and Bi-2223 at low temperature with operatingcurrent 35A. Theoretical optimization method of normal copper which conforms tothe Wiedemann-Franz law is adopted to obtain the best geometric parameterΓ.Numerical method is also used to optimize the copper current with thetemperature-dependent thermal conductivity and electric resistance. The results fromthe two methods are different arise from dealing with the thermal conductivity and theelectric resistance. The value ofΓand the heat leak from the numerical method is less than those from the theoretical method for this system, and the temperaturedistribution is also different. There is a reasonable range of length of HTS current leadto reduce the impact on the refrigerator power per unit current and the cost of HTScurrent. Given the length of the HTS current lead, there is only one best currentdensity and one best joint temperature to make the refrigerator power per unit currentminimize.3. Numerical simulation on current leads. The software ANSYS10.0 is used toanalyze the heat transfer of the current lead. the basic theory of the finite element ofheat transfer is introduced, and the finite element models are built. The relationshipbetween the heat leak at low-temperature end and the value ofΓis summarized bysolving models with different dimensions. It is found that the numerical calculationaccords well with the numerical simulation, however, the temperature distributionfrom the finite element method is different from the results from the theoreticalmethods since the different dealing ways for the material properties. Because we usethe real properties in the finite element method, the results are more exact. Numericalsimulation of the temperature distribution under various currents is investigated inorder to avoid burned off of the lead.4. Experiment on the first stage of the two-stage pulse tube cooler. In order torecover the performance of the cooler and to assure that the assembled system canoperate normally, recover experiments are performed. The results indicate that theperformance of the cooler is deteriorated and the possible reasons are analyzed, but itstill be able to meet the system requirements.